This invention relates to yield improvement in semiconductor device and more specifically to protection of individual device gates on large area metal-insulator-semiconductor devices during all stages of processing.
For purposes of this disclosure, a metal-insulator-semiconductor device denotes a device which has a gate separated from a semiconductor by an insulator. The gate is typically metal, but may be of other material, such as polysilicon.
Large area MIS arrays, such as virtual phase charge-coupled device arrays, are susceptible to static charge buildup and subsequent catastrophic discharge during various phases of the fabrication process. The static discharge can be of sufficient energy to damage, or even destroy, the dielectric layer between the gate and the corresponding opposite electrode, if the breakdown point of the material in the layer is reached or exceeded.
This build-up of static charge is unavoidable since the large area device gate structure acts as a capacitor. The large capacitance represented by the gate can store enough energy to cause the damage to the dielectric layer described above. When the accumulated potential reaches or exceeds the breakdown point of the dielectric, the discharge usually occurs in a localized area, typically the weakest point in the dielectric layer. The discharge of this energy through a small point in the dielectric causes structural damage to the dielectric which is not repairable. The result of this damage is a shorted gate or large dark current spot and therefore, a defective device.
An example of a typical operation which would cause the static build-up is the ion-implantation process. A typical dosage of an implant may cause charging of the gate, relative to the substrate, to a field strength exceeding the dielectric strength of the dielectric layer.
Prior to the present invention, gate protection from static charge build-up was typically confined to the process steps coming after the metallization step which connected the gates to protection devices such resistors or diodes, usually through metal bus lines.
It is therefore an object of this invention to provide at the time of gate formation, a structure which has a feature that will bleed off the harmful energy build-up or discharge it through an already damaged area on the edge of the slice or where contact is made to the holder, during ion implantation or other process steps.